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ASHRAE WILL GIVE YOU
THE WORLD
This ASHRAE Distinguished Lecturer is brought to you by the Society Chapter Technology Transfer Committee
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Julian R. de Bullet
ASHRAE Distinguished Lecturer
President, deBullet Consulting
703-483-0179
PLEASE MUTE CELL PHONES
VARIABLE FLOW
CHILLER PLANT DESIGN
Why Optimized Design?
• Buildings In The US Consume 39% Of Our Total Energy
• 70% Of Our Electricity Annually
• 5 Billion Gallons Potable Water Per Day For Toilets
• Typical Construction Generates 2.5 lbs. Of Solid Waste Per Square Foot
• High Performance Building Practices Can Reduce These Negative Environmental Impacts
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Chiller Basics
• Air, Water Or Evaporatively
Cooled
• Reciprocating, Scroll, Screw
Or Centrifugal Compressors
• DX or Flooded Evaporators
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Cooling Tower Basics
Induced Draft Tower
Forced Draft Tower
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Load Basics
• Chilled Water Coils
Transfer Heat From
Building Air To Chilled
Water
• Process Loads
– Cooling Jackets
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HVAC Controls
BACnetAS
H R
E
A ™
OPEN PROTOCOL IS A NORM!
Web-based?
Constant Volume Chilled Water Flow Design
Chilled Water Pump
(constant speed)
3 Way valve
Constant Volume Chilled Water Flow Design
Chilled Water Pump
(constant speed)
No valve
Commonly
called a “Wild
Coil”
Constant Volume Chilled Water Flow Design
Chilled Water Pump
(constant speed)
3 Way valve
Temperature Differential on
the chilled water supply and
return
Capacity measurement
55f
45f
50f
Constant Volume Chilled Water Flow Design
Constant chilled water flow required because of the three way bypass valves on
each air handler
Multiple chiller applications require the chillers to be in series for capacity reduction
Capacity measurement is with the chilled water temperature differential
Advantages to this design-
Ease of control
Disadvantages to this design-
Increased installation cost because of parallel piping required to allow for service
or isolation of one unit
Low fluid velocities
Chillers with different lift and mass flow capabilities
Matching or balancing the load of two chillers
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Full Load Vs. Annual Load
Design Performance
Chiller
58%
Tower
5%
Fans
24%Pumps
13%
Annual Energy Usage
Pumps
22%
Tower
2%
Chiller
33%Fans
43%
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• Variable Flow Required For Systems Over 10
HP (6.4.3.1)
– Modulate Down To 50%
• Exceptions
– Where Minimum Flow Is Less Than Flow Required
By Equipment And < 75HP
• Individual Variable Flow Pumps > 100 Feet And 50 hp Motor
– 30% Design Wattage At 50% Flow
– Controlled As A Function Of Flow Or Pressure
Differential
HVAC - Prescriptive Method (Section 6)
Hydronic Systems (6.3.4)!!!!!
Primary/Secondary Chilled Water Flow Design
Constant Flow Primary
Chilled Water Pumps
Check Valve in the Bridge or
De-coupler Circuit
Variable Flow
Secondary Chilled
Water Pump
2 Way modulating
CHW valve
Primary/Secondary Chilled Water Flow Design
Measurement devices required
for system capacity
Bridge or De-coupler temperature sensor
or bi-directional flow meter
Secondary Chilled
Water Supply
Temperature
Secondary Chilled
Water Return
Temperature
Secondary
Chilled
Water Flow
Meter
Pressure
differential
transmitter
Primary/Secondary Chilled Water Flow Design
Example of 60% system load
with 40% bypass
40% flow
42 deg f
60% valve
position
48 deg f
52 deg f
42 deg f
60% of chilled
water flow
ON
ON
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Primary/Secondary Design
• Pros
• Variable Flow Through Secondary Loop
• Stable Constant Flow through Chillers
• Cons
• Complexity
• Low Delta T Syndrome
• Stepped Primary Flow
– (More Pump Work Than Variable Primary Flow)
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Low Delta T-Definition
A Condition Whereby a Low Chilled Water
Return Temperature Causes an Excessive
Amount of Chilled Water to Circulate to
Meet System Cooling Loads and Chillers
Receiving the Low Temperature CHR
CANNOT be Loaded to Their Design
Capacity
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Low Delta T Syndrome
Low Delta T Symptoms And Solutions
• 3 Way Valves
– Don’t Use Them
• Supply Air Set points Lowered Beyond Design
– Valves Go Wide Open - No Control
– Ensure Valves Are Tracking
• Valves Not Closed When Not Required
– Ensure Valves Close When AHU Not In Use
• All System Components Not Designed For Same
Delta T
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Low Delta T Syndrome
Low Delta T Symptoms And Solutions Cont’d
• Coils And Valves Not Properly Selected
– Select Correctly
• Coils Piped “Backwards”
– Coils Must Be Piped So Water Is Counterflow To Air
• Improper Tertiary Piping
– Ensure Tertiary Setpoint Is Above Chilled Water Setpoint
• Dirty Coils- Clean the Coils
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Low Delta T Syndrome
Low Delta T Retrofit Solutions Cont’d
• VFD Chillers
– Excellent Part Load Performance Allows Two Chillers To
Operate More Efficiently Even With Parasitic Losses
• Oversize Primary Pumps
– Oversized Primary Pumps With VFDs Can Over Pump
Chillers And Avoid Starting Additional Machines
• Variable Primary Flow
– Easily Accommodates Over Pumping Chillers
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Low Delta T Syndrome
Low Delta T Retrofit Solutions Cont’d
• Reduce Delta T On Primary Side
– Larger Pumps And Piping Will Increase Capital Cost
– Penalty At Full Load
• Add Flow Control Valves At Each Coil
– Ensures Terminal Device Doesn’t Exceed Design Flow
– Space Cooling Not Satisfied
– Increase System Pressure Drop
– Adds Cost
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Low Delta T Syndrome
• Check Valve Puts Pumps In Series
• Potentially Over-pump Chiller
• Can Starve Building
• Doesn’t Fix Real Problem
Low Delta T Retrofit Solutions ?
Check Valve In
Decoupler
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Variable Primary Flow Design
General
– Primary Pump Operates When Chilled Water
Required
– Condenser Pump And Tower Operate When
Chiller Operates
– 2 Way Valves
• Diversity To Flow
– Works With Single, Series And Parallel
Chillers
Variable Primary Chilled Water Flow Design
Measurement devices required to
control the chillers
Minimum flow bypass
2 Way modulating
CHW valve
Pressure differential transmitter across each chiller
evaporator to operate the minimum flow bypass
Variable Primary Chilled Water Flow Design
Measurement devices required to
control the chillers
Minimum flow bypass
2 Way modulating
CHW valve
Pressure differential transmitter across the
chilled water header to control the chilled
water pump speed
Variable Primary Chilled Water Flow Design
Measurement devices required to
control the chillers
Minimum flow bypass
2 Way modulating
CHW valve
Chilled water flow meter to measure the total
chilled water flow
Variable Primary Chilled Water Flow Design
Example of the system at full load
Minimum flow bypass
100% open
ON
ON
42 deg F 52 deg F
CHWP’s at
100% speed
42 deg F 52 deg F
0% Open
Variable Primary Chilled Water Flow Design
Example of the system at 60% load
Minimum flow bypass
60% open
ON
ON
42 deg F 52 deg F
CHWP’s at
60% speed
42 deg F 52 deg F0% Open
Variable Primary Chilled Water Flow Design
Example of the system at 20% load
Minimum flow bypass
OFF
ON
42 deg F 52 deg F
CHWP at
25% speed
42 deg F 52 deg F5%
20%
flow
Flow Meter
Control Valve, sized for minimum chiller flow
Automatic Isolation Valves
Variable Speed Primary Pumps
Bypass
350 Ton
1,231 kW
1050 gpm
(66 l/s)
700 tons / 2 chillers = 350 tons (1,231 kW) per chiller
When building 100% loaded, entering condenser water = 85F (29.4 C)
350 Ton
1,231 kW
2 350 TON, VARIABLE PRIMARY
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Building Load 100% (700 Tons, 2462 kW)
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Variable Primary Flow Design
• Design Flowrate Determined by Tube Velocity
– Minimum 1.5 FPS ( Based On A Reynolds # Of 7500)
– Maximum 12 FPS
• At Typical Conditions, 6-7 FPS
• Select Evaporator With More Passes & Higher
Pressure Drop
• Minimum Flow Typically 50% Or Less Of Design
• Bypass Must Be Sized To Maintain Minimum Flow
Rate Of Largest Chiller
Benefits of a variable primary chilled water flow design
•Reduced first cost with the elimination of the secondary
chilled water pumps
•Reduced energy cost by the elimination of the constant
speed energy on the primary chilled water pumps
•Measurement of capacity with chilled water flow and
chilled water temperature differential
•Chillers do not operate in an unloaded condition to
generate enough chilled water flow to prevent negative
water flow through the de-coupler in a primary secondary
design
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When to consider VPF ?
When:
•System flow can be reduced by at least 30% of design.
•Design affords greater cost savings than a “de-coupled” system.
•Operators will understand how the system works and will run it properly.
•The system can tolerate a modest variation in supply water temperature.
•A single chiller is being replaced and the primary flow can be varied.
•Variable Flow can be applied to Parallel as well as Series Flow.
CHILLER STAGING
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Condenser piping not shown
Bypass piping, isolation valves not shownT2 T1T3
CH-1 Lead CH-2 Lag
CH-1 Programmed as lead chiller, CH-2 programmed as lag chiller.
CH-1 loads up until compressor speed indicates 2nd chiller appropriate
Based on compressor speed, CH-1 commands CH-2 to turn on.
Since chillers are in series, pump flows are already established.
CH-1 and CH-2 operate together to regulate T3 to set point.
At appropriate compressor speed, CH-1 commands CH-2 to turn off.
If CH-1 becomes disabled, the on board controls rotate CH-2 as the lead chiller.
CH-2 regulates to the leaving temperature T3 via the sensor.
No Human Intervention Required
Increasing system reliability in a VPF System
Does the system have the instrumentation in
place to control a VPF system?
It is critical that the system have the necessary flow meters,
differential pressure sensors, and valves in the system for
proper operation.
In addition, these devices should be selected for the system
requirements. The accuracy of the instrumentation should be
factored into the recommended minimum/maximum water flow limits.
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VPF vs. Primary Secondary
0
50000
100000
150000
200000
250000
300000
350000
400000
450000
500000
Chillers Pumps Towers Fans
kW
h
Variable Primary Flow 2 Chiller Primary/Secondary Flow 2 Chiller Parallel Flow
According to an ARTI study results, variable
primary-only chilled water systems reduced
the total annual plant energy by 3 to 8 percent,
first cost by 4 to 8 percent, and life cycle cost
by 3 to 5 percent relative to conventional
constant primary flow/variable secondary flow
chilled water systems.
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More or Less?
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Future Goals
• Net- Zero Energy Buildings
– Combination of Rising Energy Prices
– Improving Technology
– Concerns about Climate Change
• Future Goal of Net-Zero Carbon Emissions
• ASHRAE Standard 189, Design of High Performance
Green Buildings
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